Fill level measuring devices working according to the travel time principle are applied in a large number of branches of industry, e.g. in the processing industry, in the chemicals industry and in the foods industry.
In the case of fill level measurement, short transmission signals, e.g. microwaves or ultrasonic waves, are sent periodically by means of an antenna toward the surface of a fill substance and their signal components reflected on the surface are received back after a travel time dependent on the traveled path. An echo function representing the echo amplitudes as a function of travel time is formed. Each value of this echo function corresponds to the amplitude of the signal fractions reflected at a certain separation from the antenna.
That maximum of the echo function is determined, which probably is attributable to a reflection of the transmission signal on the surface of the fill substance. In such case, it is assumed, as a rule, that this maximum, subsequently referred to as the fill-level echo, has a greater amplitude than the remaining maxima of the same echo function. There results directly from the travel time of the fill-level echo, in the case of a fixed propagation velocity of the transmission signals, the separation between the surface of the fill substance and the antenna.
This conventional procedure functions without problem in a large number of applications. Problems occur, however, when the fill-level echo cannot be identified without there being some doubt as to the correctness of the identification. This can be the case, for example, when there are located in the container disturbances, such as, for example, installed objects, which reflect the transmission signals better than the surface of the fill substance.
In such cases, the current fill level can be specified once, e.g. at start-up of the fill-level measuring device. The fill-level measuring device can based on the predetermined fill level identify the maximum attributable to such as fill-level echo and track that, e.g. using a suitable algorithm. In such case, e.g. in each measuring cycle, maxima of the echo function are determined and, based on knowledge of the fill level ascertained in the preceding measuring cycle and an application-specific maximum expected rate of change of the fill level, a travel time range ascertained, in which the fill-level echo is to be expected in the current echo function. Then, based on the travel time of the maximum arising in this travel time range in the current echo function, the current fill level is determined.
Such echo tracking has the disadvantage that the fill-level echo must be tracked without interruption. When the fill-level measuring device is taken out of operation, e.g. for maintenance purposes, the fill-level echo can then not be further tracked. The fill-level measuring device is then upon restart in given cases not able to detect the fill-level echo reliably. The current fill level must again be predetermined. This can, however, in given cases mean considerable effort. When e.g. no alternative measuring methods are available, it can be necessary to empty the container completely or to fill it completely or up to a reference mark. This means in an industrial environment frequently an interruption of a manufacturing process and can be associated with considerable time consumed and high costs.
For overcoming these problems, a method is proposed in German patent, DE 10,260,962 A1 for measuring a fill level of a fill substance in a container with a fill-level measuring device working according to the travel time principle, wherein in measurement operation transmission signals are sent toward the fill substance in the container and based on their signal fractions reflected back in the container echo functions are derived, which give the amplitudes of the signal fractions as a function of their travel time, wherein following a first start-up of the fill-level measuring device there is created and subsequently used for fill level measurement a table,                whose rows correspond to discrete fill levels and whose columns correspond to discrete travel times,        in which information derived from echo functions derived following start-up concerning travel times of echo function maxima attributed to reflections on reflectors located in the container are stored as measurement points, in each case, in that row, whose row index corresponds to the associated fill level.        
The table is generated in such case, for example, by predetermining once at the beginning the current fill level, identifying the fill-level echo based on this predetermined known fill level, and tracking its time development based on the subsequently recorded echo functions by means of an echo tracking method over a time range, in which the fill level passes through the total fill level measuring range. Therewith, the subsequently recorded echo functions can based on the travel time of the tracked fill-level echo contained therein, be associated, in each case, with a fill level and information derived from the respective echo functions entered in the table as measurement points. Generated as derived information can be, for example, measurement points, which give amplitude and travel time or also only the travel time of the maxima of the echo functions in connection with the respectively associated fill level.
Alternatively, the table can also be generated without specifying a current fill level. In this case, the fill-level measuring device measures the fill level in conventional manner, takes into consideration the measured fill level as starting point for the above mentioned echo tracking, and the table is correspondingly developed. In that case, a plausibility check is subsequently performed based on the table, in order to assure that taken into consideration as starting point of the echo tracking was actually a fill-level echo and not, for example due to an incorrect fill level measurement, a maximum of the echo function attributable to another reflector.
For this, there are determined based on the information stored in the table elevation lines, which give, in each case, the travel times of maxima attributable to a determined reflector as a function of the fill level. If there results therefrom below the elevation line associated with the fill-level echo an elevation line section, whose travel times increase with increasing fill level, then the interpretation of the maximum used as starting point for the echo tracking of the fill-level echo is disproved, the table is thrown away, and a new table created.
Once the table has been created over the total fill level measuring range, it is then available for all following fill-level measurements. In such case, selected for the respectively currently derived echo function is that table row, which has the greatest agreement with the current echo function, and the current fill level is determined based on the table or in other manner determined and checked based on the table. In such case, for example, based on the fill level corresponding to the table row, a travel time window can be determined for the fill-level echo, and the exact fill level then determined based on the travel time of the maximum of the current echo function located in this travel time window. Alternatively or supplementally, echo tracking methods can be applied, based on which likewise a travel time window is determined, in which the fill-level echo is to be expected. In that case, the table can be applied continuously to check the identification of the tracked echo as fill-level echo.
The method thus delivers reliable results even under difficult measuring conditions. Such is true, however, only so long as the measuring conditions at the location of use remain unchanged. If the measuring conditions at the location of use change, then the stored table no longer correctly reflects the situation in the container. This can be the case, for example, when the properties of the fill substance located in the container change, especially because of chemical processes occurring in the container, a replacement of the fill substance or by adding of other fill substance components, or installed objects in the container are removed, added or changed in their position or properties.
If the former table would continue to be used unchanged, such could, depending on type of change of the measuring conditions, lead in given cases to considerable measurement errors.